Magnetic resonance imaging (MRI) involves selectively exciting spins in pre-determined controllable ways in an object to be imaged. Transmission elements, magnets, gradient coils, and other elements are controlled to produce the selective excitation through a combination of spatially varying magnetic fields and controlled application of radio frequency (RF) energy at a specific frequency determined as a function of the Larmor frequency of spins to be excited. Spatial localization in MRI may be accomplished using, for example, spatially varying gradients to encode the frequency and phase of a signal. The selective excitation facilitates distinguishing signal from one volume (e.g., voxel, slice, band) in an object being imaged from signal from another volume in the object. Distinctions may be made based on frequency, phase, and other attributes.
Receive elements (e.g., coils) receive signals produced by the selectively excited spins. The total signal received at an individual receive element may have been produced by spins in more than one volume. Computers therefore process the received signals to separate signal contributions from different volumes and to produce an image.
Parallel MRI (pMRI) facilitates acquiring MR images more quickly than non-pMRI. pMRI techniques may selectively excite two or more volumes (e.g., slices) at the same time and may acquire signal from the two or more volumes at the same time. Some pMRI techniques rely, at least partially, on a spatially distinct arrangement of signal receiver coils. Spatial variation in individual coil elements can replace spatial encoding that is conventionally achieved using spatially-varying magnetic fields. At least one pMRI technique (CAIPIRINHA) involves applying a unique phase pattern to each simultaneously acquired volume (e.g., slice). CAIPIRINHA is described in Breuer et al., Magn. Reson. Mod 2005; 53(3): 684-691. CAIPIRINHA is also described in U.S. Pat. No. 7,002,344, which is titled “Data acquisition method for accelerated magnetic resonance imaging in framework of the parallel acquisition of MRT data”. CAIPIRINHA stands for Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration.
In some parallel imaging techniques, aliasing artifacts resulting from an under-sampled acquisition are removed by means of a specialized image reconstruction algorithm. In CAIPIRINHA, the appearance of aliasing artifacts are modified during acquisition to improve the subsequent parallel image reconstruction procedure. CAIPIRINHA is a parallel multi-slice technique that is more efficient than other multi-slice parallel imaging concepts that use only a pure post-processing approach. In CAIPIRINHA, multiple slices of arbitrary thickness and distance are excited simultaneously with the use of multi-band RE pulses similar to Hadamard pulses. These data are then under-sampled, yielding superimposed slices that appear shifted with respect to each other. The shift of the aliased slices is controlled by modulating the phase of the individual slices in the multi-band excitation pulse from echo to echo. Slices that have essentially the same coil sensitivity profiles can be separated with this technique.
Conventionally, MRI and pMRI may have used Cartesian signal acquisition patterns. Some MRI has used other signal acquisition patterns including, spiral, radial, and so on. However, radial acquisition may not have been considered for use with phase shifting parallel imaging like CAIPIRINHA since there is no way to shift one slice or volume with respect to another using only phase modulation of the RF pulse. Thus prior CAIPIRINHA methods are not compatible with a radial trajectory.